1. Field of the Invention
The present invention relates to a method of treating blast furnace gas and to an apparatus for carrying out that method.
It is known to treat blast furnace gas in order to separate a coarse fraction of the gas which has a low lead and zinc concentration. After filtration such a component may be used as a raw material for feeding to a blast furnace. In known methods the coarsest dust particles in the gas are removed by a dry removal process then the gas is wet scrubbed and the grains of the resulting blast furnace gas dust are separated to obtain the desired coarse fraction. Such a method of treating blast furnace gas is described in French patent specification Nos. 2,299,089 and 2,362,669 and in published European patent application No. 0,003,464, in which a hydrocyclone is used. A hydrocyclone is a known device and its use for separating dusts according to grain size is described in e.g. U.S. Pat. No. 2,760,635. The structure of a hydrocyclone and its use in separating blast furnace gas into fractions with respectively higher and lower lead and zinc concentrations will therefore be clear to the skilled man and will not be described in detail in this specification.
The advantage of such known methods for separating blast furnace gas is that efficient separation can be achieved at low cost. However, a problem may arise in the known methods when the fraction of the blast furnace gas with small grain size is low and when the initial concentrations of lead and zinc is also low. These conditions may occur, for example, when the raw material for treatment is blast furnace gas resulting from a standard wet scrubbing process followed by the passing of the blast furnace gas through a dust bag filter. Wet scrubbing followed by dust bag filtering is common in modern blast furnace plants, the coarse material from the dust bag filter being returned directly to the blast furnace.
The assignees of the present application have analysed a large number of samples of blast furnace gas dust collected after wet scrubbing, produced by their own blast furnaces. The average lead and zinc concentration in the samples was found to be 0.67% and 0.22% respectively. It was found, however, that these concentrations may vary in dependence on the composition of the charge of the blast furnace and in the process carried out in the furnace.
Table 1 shows a typical blast furnace gas dust composition.
TABLE 1 ______________________________________ percentages percentage percentage fraction by weight of zinc of lead ______________________________________ &lt;5 .mu.m 14 3.61 1.25 5-10 3 0.64 0.18 10-20 12 0.28 0.10 20-30 7 0.20 0.04 30-60 28 0.18 0.05 &gt;60 36 0.14 0.04 ______________________________________
This material suspended in large quantities of water (e.g. 0-4 grams of dust/liter), is conveyed to the above-mentioned grain separation plant.
If this gas dust, suspended in large quantities of water (e.g. 0 to 4 grams of dust per liter) is supplied to the known treatment apparatus the top fraction of the hydrocyclone will be too coarse, unless separation is carried out on a smaller grain size in which case the separation will be inefficient and the amount of dust separated will be small.